DESCRIPTION: Verbatim from the applicant's abstract) This R21 pilot application proposes to develop novel animal models of analyzing alloreactive T cells through the generation of CD4+ T cell receptor transgenic mice specific for either donor antigen presenting cells ('direct') or alloantigens presented by host antigen-presenting cells ('indirect'). CD4+ T cells play an essential role in immunity to allogeneic pancreatic islet transplants as well as in the induction of tolerance to such grafts. The nature of CD4 T cell involvement in these immune responses is unclear but critical to determine for the progression of clinical islet transplantation for IDDM without the current need for long-term recipient immunosuppression. Potential roles of the CD4+ T cell in allograft immunity may be in the collaboration with CD8+ T cells, facilitating their ability to mediate allograft rejection or they could contribute to the production of a graft-specific antibody response by supplying help to B cells. Alternatively, CD4+ T cells can function as effector cells, sensitized to allogeneic tissues through either of two distinct pathways: 1) a 'direct' (donor MHC-restricted) pathway in which CD4 T cells interact with MHC class II antigens expressed by donor APCs or 2) an 'indirect' (host MHC-restricted) pathway in which they interact with graft antigens processed and presented by host APCs in the context of class II MHC antigens. With these varied potential functions, the specific role the CD4+ T cell plays in allograft immunity or in tolerance induction remains undetermined. Alloreactive CD8+ T cell receptor (TCR) transgenic mouse models have demonstrated tremendous value in dissecting the role of the CD8 T cell in immune responses such as positive and negative selection, tolerance to peripheral antigens, autoimmunity and tumor immunity. Currently, there are no available alloreactive CD4+ TCR transgenic mice. Therefore, the specific aims of this project are to create TCR transgenic mice bearing CD4+ T cells activated via 1) the 'direct' (donor APC-dependent) and 2) the 'indirect' (host APC-dependent) pathways of alloantigen presentation. Such alloreactive CD4+ TCR transgenic animals would supply us with a renewable source of direct and indirect effector T cells of known specificity from which role of the CD4+ T cell in established models of islet allograft rejection and tolerance could be defined. They would allow critical evaluation of the hypothesis that: Both islet allograft immunity and tolerance are dependent on CD4 T cells that recognize graft antigen through indirect host MHC-restricted antigen presentation.